This Program Project on the structural biology of HIV- and AIDS-related proteins focuses on three steps in the viral infectious cycle: membrane fusion during entry, reverse transcription, and regulation of transcriptional initiation from the LTR on integrated pro-virus. (1) The structural of the ectodomain of gp41, revealed by a crystallographic analysis carried out during the previous project period leads directly to strategies for drug discovery, using biased combinatorial methods. The target is a "snap-back" step in the conformation change that generates fusion-competent gp41. The results of initial combinatorial screen will be examined biochemically and crystallographically. The interaction of gp120 with cellular chemokine receptors appear to be essential for triggering the fusion-activating conformation change in gp41. (2) Crystals of a template-primer complex of HIV-1 reverse transcriptase (RT), generated recently, will lead to a detailed crystallographic analysis of RT mechanism and of the basis of nucleoside analog drug resistance. The crystallographic efforts will be accompanied by work on novel approaches to biased combinatorial screens for compounds that bind at defined locations on the RT molecular surface. (3) Structural studies will be carried out on NFkappaB and SP1 bound together at sites known to be critical for transcription from integrated pro-virus. One goal is to expand understanding of combinatorial gene regulation at composite sites such as the HIV-1 LTR. Another goal is to determine whether interacting surfaces on these cellular proteins might provide suitably specific targets for screening ligands.